The present invention relates to a hybrid welding process and to a hybrid welding set combining a laser beam and an electric arc, particularly a plasma arc, and to its application to the welding of tailored blanks intended for the motor-vehicle industry.
Laser technology has been known and widely used for years to weld various metallic materials, such as workpieces made of alloy steels or non-alloy steels, coated steels, stainless steels, and aluminium and aluminium alloys. In general, a laser welding set comprises a solid-state or gas laser oscillator producing a coherent monochromatic high-energy beam, an optical path provided with deviating mirrors or else an optical fibre allowing the laser beam to be conveyed to a welding head located opposite the sheet to be welded. Conventionally, the welding head comprises a lens or one or more focussing mirrors so as to focus the laser beam onto one or more focal points in the thickness of the material to be welded and in the joint plane formed by joining together, edge to edge, the parts of the workpieces to be welded together so as to locally concentrate sufficient power density to melt the material to be welded. Usually, the welding head comprises a gas feed device for feeding with welding gas, also called assist gas, by means of a gas delivery nozzle placed coaxially with the laser beam. This gas feed device may also be external to the actual laser welding head.
An alternative solution for welding the edges of one or more workpieces to be joined together, for example the longitudinal edges of a metal sheet formed into a pipe, or of two metal workpieces brought together edge to edge, consists in melting the edges to be joined together by means of one or more electric arcs.
Some of these processes widely employed in industry use gas either as shielding gas or as active gas. In this regard, mention may be made of TIG (Tungsten Inert Gas), MIG (Metal Inert Gas), MAG (Metal Active Gas) processes or else plasma-arc processes.
In contrast, other processes do not generally use gas during the welding of the pipe, for example submerged-arc welding processes.
However, laser welding processes or arc-welding processes each have drawbacks which are specific to them.
Consequently, combined or hybrid processes have been developed which combine one or more laser beams with one or more electric arcs, particularly welding processes which combine a laser beam with an arc plasma jet, also called plasma-laser welding processes.
Various hybrid arc/laser welding processes have been described, for example in the documents EP-A-793558; EP-A-782489; EP-A-800434; U.S. Pat. No. 5,006,688; U.S. Pat. No. 5,700,989; EP-A-844042; xe2x80x9cLaser GTA Welding of aluminium alloy 5052xe2x80x9d by T. P. Diebold and C. E. Albright, 1984, pages 18-24; SU-A-1815085 and U.S. Pat. No. 4,689,466; xe2x80x9cPlasma arc augmented laser weldingxe2x80x9d by R. P. Walduck and J. Biffin, pages 172-176, 1994; or xe2x80x9cTIG or MIG arc augmented laser welding of thick mild steel platexe2x80x9d, Joining and Materials, by J. Matsuda et al., pages 31-34, 1988.
In general, the plasma/laser or more generally an arc/laser welding process is a combined or hybrid welding process which combines electric arc welding with a laser beam.
The arc/laser process consists in generating an electric arc between an electrode, which may or may not be consumable, and the workpiece to be welded, and in focusing a powerful laser beam, especially a YAG-type or CO2-type laser, in the arc zone, that is to say near or in the joint plane obtained by joining together, edge to edge, the parts to be welded together.
Such a hybrid process makes it possible to considerably improve the welding speeds compared with laser welding alone or with arc welding alone.
In addition, such a hybrid process furthermore makes it possible to appreciably increase the tolerances on positioning the workpieces before welding since laser welding alone requires high precision in positioning the parts to be welded because of the small size of the focal spot of the laser beam.
The use of a plasma/laser process, and more generally, of an an arc/laser process, requires the use of a welding head which makes it possible to combine, in a small space, the laser beam and its focusing device, and a suitable welding electrode. Several head configurations are described in the abovementioned documents and it may be stated, in summary, that the laser beam and the electric arc or plasma jet may be delivered by one and the same welding head, that is to say they leave via the same orifice, or else via two separate welding heads, one delivering the laser beam and the other the electric arc or plasma jet, the two coming together in the welding zone.
Arc/laser hybrid processes are especially suitable for welding tailored blanks for the automobile industry, since, in addition to the abovementioned advantages, they make it possible to obtain a weld bead which is well wetted and free of undercuts, as recalled in the documents EP-A-782 489 and xe2x80x9cLaser plus arc equals powerxe2x80x9d, Industrial Laser Solutions, February 1999, pages 28-30.
Such welding consists in joining together two sheets or workpieces, generally made of steel, galvanized steel, or aluminium, of different thicknesses and/or different grades. Depending on the welding methods and preparations used, the joint to be welded is conventionally characterized by a difference in level between the upper planes of each of the workpieces to be welded, thus resulting in the creation of a xe2x80x9cstepxe2x80x9d, as shown in FIG. 1.
The reverse situation may also be encountered, namely joints of the tailored-blank type in which the upper planes are aligned but the lower planes of which are not on the same level and where therefore the xe2x80x9cstepxe2x80x9d is located on the reverse side of the joint to be welded, as may be seen in FIG. 2.
Welds of this kind (FIG. 1 or FIG. 2) are often found in the motor-vehicle industry in which the workpieces thus welded are then pressed in order to give them their final shapes, for example the various workpieces which are used in the manufacture of a car body and, for example, the doors, the roof, the bonnet or the boot. They may also be found in the structural elements of the passenger compartment.
Furthermore, there also exists the case of workpieces to be welded together which are of the same thickness but of different grades from each other.
This hybrid process is also well suited to the welding of many types of joints such as, for example, the angle weld shown schematically in FIG. 3 and the lap weld shown in FIG. 4.
From the industrial standpoint, the use of such a hybrid process requires machines or robots which have either means for conveying and moving the workpieces to be welded under the plasma-laser welding head or means for moving the head itself over the workpiece to be welded.
Depending on the number of joints to be welded per workpiece and depending on the number of workpieces to be welded, these machines or robots require not only to be precisely positioned with respect to the welded joint to be produced but also, and above all, to be able to stop the welding operation intermittently.
This is because, as mentioned above, each of the workpieces to be welded may have several joints to be welded, for example joints located at different points on the workpiece in question, and it is then essential to stop the welding operation while the machine or robot positions the welding head on the next joint, or vice versa.
Likewise, the same operation must be carried out when changing new workpieces, that is to say after welding one workpiece and before welding the next workpiece.
This means that, during these welding stop operations, neither the laser beam nor the plasma arc must continue to impinge on the workpiece.
In the case of the plasma arc, preventing the arc from settling on the workpiece(s) to be welded is easily achievable in practice:
either by extinguishing the arc, that is to say by setting the welding current to zero;
or by preventing transfer of the arc onto the workpiece to be welded, that is to say by switching from a welding arc phase to a pilot arc phase during which the electric arc is established between the electrode and, for example, the inner surface of the plasma/laser welding head and therefore without any contact between the electric arc and the workpiece(s) to be welded. In general, this is achieved by greatly reducing the electric arc current (intensity) and by simultaneously setting the terminal part of the plasma/laser welding head to the potential of the workpiece to be welded.
Optionally, it is also possible, complimentarily, to reduce the gas flow rate and/or change the nature of the gas.
Thus, mention may be made of document EP-A-793 558 which recommends interrupting the electric arc and the laser beam almost simultaneously, by firstly reducing the electric current until the plasma arc has stopped and then de-exciting the laser at a selected moment after stopping the plasma arc.
However, this way of proceeding is not ideal as it is not adapted to the mass production of welded components, especially because of the time wastage and the loss of productivity that it entails.
A problem arises with the laser beam since, unlike the procedure that can be applied to the electric arc, it is not possible to stop or turn off the laser beam and then restart it or turn it back on as simply as the arc, as this results in large power instabilities when restarting it at the beginning of the next welding phase, and in time wastage and therefore loss of productivity since restarting the laser, at the beginning of the welding of the next joint, requires a relatively long time during which nothing can happen, that is to say no welding can be carried out, this time being necessary for thermally stabilizing the laser oscillator.
Thus, the present invention aims to solve this problem by providing an improved hybrid arc/laser welding process which makes it possible to avoid turning off the laser beam during the time period elapsing between the end of producing one welded joint and the start of welding the next joint, in particular in order to allow the machine or robot to position the welding head and the start of the joint plane to be produced relative to one another and/or to allow a change of new workpieces to be welded together, that is to say after welding one or more workpieces and before welding the next workpiece(s), and to do so without the laser impinging on the workpiece(s) to be welded or on the support frame supporting the workpieces while they are being welded, in order to prevent damage to the workpieces or to the unit itself.
The solution provided by the present invention is therefore a hybrid welding process and a hybrid welding unit, and the use of such a process or of such a unit in the welding of workpieces intended, in particular, for the motor-vehicle industry, such as tailored blanks.
More specifically, the invention relates to a process for welding one or more metal workpieces to be welded together by using at least one laser beam and at least one electric arc, in particular a plasma arc, in which process, after at least one welded joint has been produced on the said workpiece(s), the laser beam is sent and/or deflected into radiation absorption means allowing at least some of the radiation of the said laser beam to be absorbed.